1. Field of the Invention
This invention relates to a braking force control system for a vehicle which prevents the wheels from locking and skidding during braking.
2. Description of the Prior Art
As disclosed, for instance, in Japanese Patent Publication No. 63(1988)-41332, there has been known an antilock control system in which the braking force acting on each wheel is controlled so that the slip ratio of the wheel converges on a target slip ratio (normally a slip ratio at which the friction coefficient between the wheel and the road surface is maximized).
In such an antilock control system, when tendency of the wheel to lock during braking, the brake fluid pressure is once fixed to the value at that time, and when the tendency is further enhanced and the slip ratio of the wheel exceeds the target slip ratio, the brake fluid pressure begins to be reduced while when the slip ratio is reduced below the target slip ratio by fixing the brake fluid pressure, the brake fluid pressure begins to be increased.
FIG. 5 shows manners of change in the slip ratio after the slip ratio once begins to reduce by fixing the brake fluid pressure. In FIG. 5, the target slip ratio has a certain width between upper and lower limits. Assuming that the slip ratio increases by application of the brakes and it is determined that a wheel tends to lock and the brake fluid pressure is fixed at time t1, the slip ratio will not fall below the lower limit of the target slip ratio and will remain in the target slip ratio range as shown by line A so long as the friction coefficient .mu. of the road surface is small. On the other hand, when the friction coefficient .mu. of the road surface is not small, the slip ratio continues to fall and becomes smaller than the lower limit of the target slip ratio range at time t2. In the latter case, the brake fluid pressure begins to be increased at the time t2 at a predetermined rate to return the slip ratio to the target slip ratio range, and as a result, sometimes the slip ratio does not fall greatly and returns to the target slip ratio range in a short time as shown by line B or falls greatly and takes a long time to return to the target slip ratio range as shown by line C (shortage in rise of slip ratio).
When the friction coefficient .mu. of the road surface is large, the slip ratio sometimes changes along the line C. That is, when the slip ratio temporarily increases during braking on a large .mu. road (road having a large friction coefficient .mu.), for instance, due to snow which remains on a part of the road, it is determined that the wheel tends to lock and the brake fluid pressure is fixed. In such a case, the friction coefficient .mu. of the road surface soon returns to being large and accordingly the slip ratio begins to fall greatly and does not return to the target slip ratio range in a short time even if the brake fluid pressure is increased at the predetermined rate from that time. When such a shortage in the rise of the slip ratio occurs, the deceleration of the vehicle body lowers markedly and the vehicle may skid on some road surfaces. Accordingly, in such a case, it is preferred that teh brake fluid pressure be increased at a higher rate after the slip ratio falls below the lower limit of the target slip ratio range in order to quickly increase the slip ratio.
However if the rate at which the brake fluid pressure is increased after the slip ratio falls below the lower limit of the target slip ratio range is increased indiscriminately, there arises another problem, though the above problem can be overcome, that, in the case where the slip ratio changes as shown by the line B, the slip ratio quickly increases and can exceed the upper limit of the target slip ratio range by a large amount even if the brake fluid pressure is fixed at the time the slip ratio exceeds the lower limit of the target slip ratio range.